Radar device

ABSTRACT

A radar device including: a reception antenna that receives radio waves, includes at least three antennas and is arranged so that the phase center points thereof form an isosceles triangle; an arrival direction detection unit that detects an arrival direction of the radio waves by a phase monopulse method; and a phase correction unit that corrects a phase difference between phases of radio waves respectively received by two adjacent antennas among the three antennas based on the relationship of the phases of the radio waves respectively received by the three antennas.

The disclosure of Japanese Patent Application No. 2008-292349 filed onNov. 14, 2008, including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a radar device of the phase-monopulse type thatdetects objects.

2. Description of the Related Art

Radar has conventionally been installed in vehicles to detect distance,relative speed and direction of obstacles (such as in Japanese PatentNo. 3433417 or Japanese Patent Application Publication No. 2001-166029(JP-A-2001-166029)).

Japanese Patent No. 3433417 discloses a phase monopulse radar devicecapable of detecting objects in the vertical direction by arrangingarray antennas, in which element antennas that receive radio wavesreflected by an object are arranged in the longitudinal direction,horizontally while shifting in the vertical direction. According to thisdevice, since a vehicle can distinguish a road or a sign on a road,erroneous detection of an object can be prevented. In addition,JP-A-2001-166029 discloses digital beamforming (DBF) radar.

An explanation of a method for calculating the arrival direction ofradio waves using a phase monopulse system is explained with referenceto FIGS. 4A and 4B. FIG. 4A indicates the relationship between thearrival direction of radio waves and the difference in path length ofthe radio waves in the case of receiving radio waves with two receptionantennas. Phases of the reception signals of the two reception antennasare different form each other due to the path length difference of theradio waves. FIG. 4B indicates the relationship between radio wavearrival angle (horizontal axis, [deg]) and phase difference (verticalaxis, [deg]) in the case of installing reception antennas horizontallyand using a value of λ/2 for the antenna interval d. Once the phaseangle Δφ has been determined, the arrival direction of the radio wavescan be calculated based on the formula θ=arcsin (λ·Δφ/(2·π·d)).

However, since detection of the phase difference of radio waves iscarried out by detecting periodic shifts in the waveform of the radiowaves, phase difference can only be detected within the range of −π to π[rad] due to this periodicity. Even if there is a phase difference thatexceeds this range, it ends up being observed as a phase differencebetween −π to π[rad] (and this state is referred to as the occurrence ofphase aliasing). Thus, it was conventionally considered necessary tomake the antenna interval smaller than the half-wavelength of thewavelength of a carrier wave in order to be able to uniquely specify thearrival direction of radio waves (see FIG. 4B). However, in order toprevent exacerbation of characteristics attributable to physicaldimensions of the antenna element and inter-antenna coupling, it is notpossible to make the antenna interval as small as possible, but ratherthe antenna interval is frequently made to be half of the wavelength λ.

In the case of detecting the arrival direction of radio waves by a phasemonopulse system, the azimuth angle at which radio waves arrive can bedetected in the case of arranging two reception antennas in a rowhorizontally. Similarly, the elevation angle at which radio waves arrivecan be detected in the case of arranging two reception antennas in a rowvertically. Thus, both the azimuth angle and elevation angle at whichradio waves arrive can be detected by arranging three or more receptionantennas two-dimensionally (namely, such that all of the receptionantennas are not arranged in a straight line).

However, in the case of detecting objects around a vehicle, since theelevation angle range in which detection targets are present is limited,the antenna directionality pattern thereof is preferably wider in thedirection of azimuth angle and narrower in the direction of elevationangle. Since array antennas are typically long in the vertical directionand short in the horizontal direction, an antenna arrangement thatrealizes an antenna directionality pattern as described above has a lowdegree of freedom. For example, in the case of arranging two rectangularantennas along the lengthwise direction of the antennas, the intervalbetween the phase centers of the antennas becomes excessively wide, andthe angle range over which the arrival direction of radio waves can beaccurately detected using a phase monopulse system decreases. Inaddition, there is also the disadvantage of the dimensions in thevertical direction of the antenna housing becoming excessively large.Thus, in the case of detecting the arrival direction of radio waves witha phase monopulse system using antennas, it is essential to arrange theantennas in the direction perpendicular to the lengthwise direction ofthe antennas. For this reason, it has been difficult to detect both theazimuth angle and elevation angle at which radio waves arrive with aphase monopulse method using a plurality of antennas.

SUMMARY OF THE INVENTION

Therefore, the invention provides a radar device capable of detectingboth azimuth angle and elevation angle at which radio waves arrive by aphase monopulse method using a plurality of antennas.

A first aspect of the invention is a radar device, including: areception antenna that receives radio waves, is constituted by at leastthree antennas and is arranged so that phase center points thereof forman isosceles triangle; an arrival direction detection unit that detectsan arrival direction of the radio waves by a phase monopulse method; anda phase correction unit that corrects a phase difference between phasesof radio waves respectively received by two adjacent antennas among thethree antennas based on a relationship of phases of radio wavesrespectively received by the three antennas.

According to this configuration, both azimuth angle and elevation angleat which radio waves arrive can be detected by a phase monopulse methodusing a plurality of antennas.

A distance between the phase center point of the center antenna amongthe three antennas and the phase center point of at least one of theremaining two antennas may be made to be larger than one-half thewavelength λ of the radio waves.

Since the phase difference of radio waves received by these antennas canbe corrected by the phase correction unit even if the distance betweenthe phase center points of two adjacent antennas is larger than one-halfthe wavelength λ of the radio waves, the degree of freedom ofarrangement of the antennas increases.

The reception antenna may be configured such that a first distancebetween the phase center point of the center antenna among the threeantennas and the phase center points of the remaining two antennas in adirection along the base of the isosceles triangle is one-half or lessthe wavelength λ of the radio waves, a second distance between the phasecenter point of the center antenna and the phase center points of theremaining two antennas in a direction perpendicular to the base of theisosceles triangle is one-fourth or less the wavelength λ of the radiowaves, and in the case the absolute value of the sum of a first phasedifference, which is the phase difference between a phase of radio wavesreceived by one of the remaining two antennas among the three antennasand a phase of radio waves received by the center antenna, and a secondphase difference, which is the phase difference between a phase of radiowaves received by the other of the remaining two antennas and the phaseof the radio waves received by the center antenna, is larger than2×(second distance)×2π/λ, the phase correction unit may correct one ofthe first phase difference and the second phase difference, whoseabsolute value is larger than that of the other of the first phasedifference and the second phase difference.

According to this configuration, the phase difference among the firstphase difference and the second phase difference which is shifted fromthe true phase difference can be specified.

In the case of correcting one of the first phase difference and thesecond phase difference, whose absolute value is larger than that of theother of the first phase difference and the second phase difference, thephase correction unit may correct the phase difference by subtracting 2πfrom the phase difference if the value of the phase difference ispositive, and may correct the phase difference by adding 23π to thephase difference if the value of the phase difference is negative.

According to this configuration, the phase difference among the firstphase difference and the second phase difference which is shifted fromthe true phase difference can be corrected.

The direction along the base of the isosceles triangle may besubstantially a horizontal direction.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and further objects, features and advantages of theinvention will become apparent from the following description of exampleembodiments with reference to the accompanying drawings, wherein likenumerals are used to represent like elements and wherein:

FIG. 1 is a block diagram of an antenna of a radar device as describedin an embodiment of the invention;

FIG. 2 shows the relationship of phase difference at each phase centerof a radar device as described in an embodiment of the invention;

FIG. 3 is a flow chart of phase correction processing as described in anembodiment of the invention; and

FIGS. 4A and 4B show the relationship between phase difference and anglein a phase monopulse system.

DETAILED DESCRIPTION OF EMBODIMENTS

The following provides an explanation of an embodiment of the device asdescribed in the invention with reference to the drawings.

FIG. 1 is a block diagram of an antenna 10 of a radar device asdescribed in an embodiment of the invention. The antenna 10 is arranged,for example, facing towards the front or rear of a vehicle. The antenna10 is installed, for example, so that the axial direction thereof is thevertical direction. The antenna 10 is provided with a transmissionantenna 11 and a reception antenna 3. The transmission antenna 11 isprovided with a plurality of transmission element antennas 111 on thesame plane, and

The reception antenna 3 is provided with array antennas 31 to 33. Thearray antennas 31 to 33 are respectively composed by arranging aplurality of reception element antennas 30 in a row within the sameplane in the vertical direction. The array antennas 31 to 33 arearranged horizontally within the same plane in the order of arrayantenna 32, 31 and 33. The array antenna 31 functions as the “centerantenna” of the invention. Radio waves radiated by the transmissionantenna 11 are reflected by an object and received by the receptionelement antennas 30.

Phase center points 311, 321 and 331 of the array antennas 31 to 33 arelocated nearly in the center of each of the array antennas 31 to 33.Array antennas 31 to 33 are arranged by shifting the location of thephase center point of the center array antenna 31 in the verticaldirection with respect to the other array antennas so that the phasecenter points 311, 321 and 331 form an isosceles triangle overall. Here,this isosceles triangle is made to be such that the distance between thephase center point 311 and the phase center point 321 and the distancebetween the phase center point 311 and the phase center point 331 areequal. As a result of arranging in this manner, the direction in whichradio waves arrive can be detected based on the phase differencesbetween phases of each radio wave received by the array antennas 31 to33 with respect to not only the horizontal direction but also thevertical direction. The reception antenna 3 is able to detect thearrival direction of radio waves, namely the direction in which anobject (such as an obstacle) is present (horizontal and verticaldirections) based on the phase differences.

Furthermore, in the following explanation, the interval in thehorizontal direction between the phase center point 311 and the phasecenter point 321 as well as the interval in the horizontal directionbetween the phase center point 311 and the phase center point 331 aredesignated as α, while the interval in the vertical direction betweenthe phase center point 311 and the phase center 321 point as well as theinterval in the vertical direction between the phase center point 311and the phase center point 331 are designated as β as shown in FIG. 1.This embodiment is premised on the above-mentioned a being one-half orless the radio wave wavelength λ (α≦λ/2), and the above-mentioned βbeing one-fourth or less the radio wave wavelength λ(β≦λ/4).

Next, the following provides an explanation of the relationship of thephases of radio waves received by each of the array antennas 31 to 33 ofthe reception antenna 3 using FIG. 2. The three black dots in FIG. 2represent the positional relationship of the phase center points 311,321 and 331 shown in FIG. 1. An equiphase plane 37 is a plane in whicheach of the phases of radio waves arriving from a prescribed radio wavearrival direction 36 is equal. Each of the four broken linesperpendicular to the radio wave arrival direction 36 shown in FIG. 2 isan equiphase plane 37. Although the equiphase planes 37 are actuallyplanes, they are represented with straight lines in FIG. 2 for the sakeof convenience.

Each of the radio waves received by the array antennas 31 to 33 is inputto a signal processing unit not shown. The phases ψ1, ψ2 and ψ3represent the phases of radio waves received at each phase center point311, 321 and 331. The signal processing unit calculates the phasedifference ψ21 (=ψ2−ψ1) and the phase difference ψ31 (=ψ3−ψ1) based onthe phases ψ1, ψ2 and ψ3.

The values of the phase differences ψ21 and ψ31 are inherently notnecessarily values from −π to π, but rather n·2π+p (where, “•” is asymbol that indicates multiplication, n is an integer and −π<p<π).However, the phase differences ψ21 and ψ31 are represented with valueswithin the range of −π to π, and are used in processing to be describedlater.

A virtual antennas is placed in the center of the phase center points331 and 321, and the phase ψ1′ represents the phase of radio wavesreceived at that phase center point. At this time, a phase difference ψVin the vertical direction and a phase difference ψH in the horizontaldirection between phases of radio waves respectively received by twoadjacent allay antennas can be determined according to the followingformulas: ψV=ψ1′−ψ1=((ψ2−ψ1)+(ψ3−ψ1))/2=(ψ21+ψ31)/2 (Formula 1); andψH=ψ2−ψ1′=((ψ2−ψ1)−(ψ3−ψ1))/2=(ψ21−ψ31)/2 (Formula 2).

Here, although the phase differences ψ21 and ψ31 are required to be truein order to correctly calculate the phase differences ψV and ψH, thereis the possibility of the phase differences ψ21 and ψ31 shifting by anintegral multiple of 2π in either the positive direction or negativedirection from the true values thereof. Therefore, the signal processingunit suitably corrects the values of the phase differences ψ21 and ψ31.The signal processing unit functions as the “phase correction unit” and“arrival direction detection unit” of the invention.

(Correction Method of Phase Differences ψ21 and ψ31) The followingprovides an explanation of a method for correcting the phase differencesψ21 and ψ31 using FIG. 2. The phase differences ψ21 and ψ31 arecorrected according to whether or not the phase difference ψV in thevertical direction as determined based on the phase difference ψ21 andthe phase difference ψ31 is a value within the range of possible values,which is determined based on the interval β in the vertical directionbetween the phase center point 311 and the phase center points 321,331.

In the case both the phase differences ψ21 and ψ31 are true, since ψVcan only be a value that is within the range of −β·(2π/λ) to β(2π/λ),the expression |(ψ21+ψ31)/2|≦β(2π/λ), namely |ψ21+ψ31|≦2·β(2π/λ)(Formula 3) ought to be valid from the above-mentioned Formula 1. Here,since β≦λ/4 according to the previously stated premise, possible valuesof ψ21+ψ31 are within the range of −π to π. However, in the case eitherone of the phase differences ψ21 and ψ31 has shifted by an integralmultiple of 2π from the true value, the value of ψ21+ψ31 also shifts byan integral multiple of 2π from the true value, and the Formula 3 is nolonger satisfied. On the basis thereof, in the case the Formula 3 is notsatisfied, at least one of either the phase difference ψ21 and the phasedifference ψ31 can be determined to have shifted from the true value.

However, as long as α is equal to or less than λ/2 (α≦λ/2) and β isequal to or less than λ/4(β≦λ/4) as indicated by the previously statedpremise, the possibility of both the phase differences ψ21 and ψ31shifting from the true values is zero. Thus, if a phase difference isassumed to have shifted, only one of either the phase difference ψ21 orψ31 has been shifted. In addition, as long as α is equal to or less thanλ/2 (α≦λ/2) and β is equal to or less than λ/4 (β≦/4), there is nopossibility of the phase difference ψ21 or ψ31 shifting by 4π or more.Thus, if a phase difference is assumed to have shifted, the shift isonly 2π in the positive direction or negative direction from the truevalue.

In the case it is determined from the above-mentioned Formula 3 thateither one of the phase difference ψ21 or ψ31 has shifted from the truevalue, one of the phase difference ψ21 and the phase difference ψ31whose absolute value is larger than that of the other is determined tobe the false value that has shifted from the true value. A phasedifference shifts from the true value by 2π in the case the true valuehas a value outside the range of −π to π. In addition, |ψ21−ψ31| isequal to or less than 2π(|ψ21−ψ31|≦2π) as a result of setting α to avalue equal to or less than λ/2 (α≦λ/2). Thus, one of the phasedifference ψ21 and the phase difference ψ31 whose absolute value islarger than that of the other can be determined to have an false value.

In the case the true value has a value outside the range of −π to π, thefalse value is a value that has shifted by 2π from the true value andthe sign of that value has inverted. For example, The false value is π−γif the true value is −π−γ, and the sign of −π−γ differs from the sign ofπ−γ since the value of γ cannot be larger than π according to thepreviously stated premise. Thus, in the case the value of a phasedifference among the phase differences ψ21 and ψ31 that has beendetermined to be false is a positive value, that value can be correctedto the true value by subtracting 2π from the value of that phasedifference. Similarly, in the case the value of a phase difference thathas been determined to be false is a negative value, that value can becorrected to the true value by adding 2π to the value of that phasedifference.

Next, an example is indicated of phase correction processing carried outby the above-mentioned signal processing unit using the flow chart ofFIG. 3.

In ST1, the signal processing unit acquires phase information (phasesψ1, ψ2 and ψ3) of radio waves received by the array antennas 31 to 33.

In ST2, the signal processing unit calculates the phase difference ψ21(=ψ2−ψ1) and the phase difference ψ31 (=ψ3−ψ1).

In ST3, the signal processing unit determines whether or not therelationship of |ψ21+ψ31|>2·β·(2π/λ) is satisfied. If the result of thisdetermination is affirmative (YES), then one of either of the phasedifferences ψ21 and ψ31 is false, and the signal processing unit carriesout correction processing starting in ST4. If the result of thisdetermination is negative (NO), since both of the phase differences ψ21and ψ31 are true, processing ends without correcting the phasedifferences ψ21 and ψ31.

In ST4, the signal processing unit determines whether or not theabsolute value of the phase difference ψ21 is larger than the absolutevalue of the phase difference ψ31. If the result of this determinationis affirmative (YES), the flow proceeds to ST5. If the result of thisdetermination is negative (NO), the flow proceeds to ST8.

In ST5, the signal processing unit determines whether or not the phasedifference ψ21 is greater than zero. In the case the phase differenceψ21 is greater than zero (YES in ST5), the signal processing unitcorrects the phase difference ψ21 and uses the value of ψ21−2π as thetrue value of the phase difference ψ21 (ST6). In the case the phasedifference ψ21 is zero or less (NO in ST5), the signal processing unitcorrects the phase difference ψ21 and uses the value of ψ21+2π as thetrue value of the phase difference ψ21 (ST7).

In ST8, the signal processing unit determines whether or not the phasedifference ψ31 is greater than zero. In the case the phase differenceψ31 is greater than zero (YES in ST8), the signal processing unitcorrects the phase difference ψ31 and uses the value of ψ31−2π as thetrue value of the phase difference ψ31 (ST9). In the case the phasedifference ψ31 is zero or less (NO in ST8), the signal processing unitcorrects the phase difference ψ31 and uses the value of ψ31+2π as thetrue value of the phase difference ψ31 (ST10).

(Calculation of Radio Wave Arrival Direction) Next, an explanation isprovided of the method used to calculate the arrival direction(elevation and azimuth angles) of radio waves. The signal processingunit calculates the previously described phase differences ψV and ψHaccording to the above-mentioned Formulas 1 and 2 based on the truevalues of the phase differences ψ21 and ψ31 corrected in the mannerdescribed above.

Next, the signal processing unit determines the azimuth angle φ and theelevation angle θ of the radio wave arrival direction 36 using the phasedifferences ψV and ψH calculated in the manner described above. Here,when the azimuth angle φ and the elevation angle θ are calculated byapplying the phase differences ψV and ψH to a phase monopulsecalculation formula, the values of the azimuth angle φ and the elevationangle θ can be calculated as shown below.θ=arcsin(λ·ψV/(2·π·β))  (Formula 4)φ=arcsin(λ·ψH/(2·πα·cos θ))  (Formula 5)

The following provides a supplementary explanation of the aboveexamples.

In FIG. 1, the reception antenna 3 may be provided with array antennasin addition to the array antennas 31, 32 and 33. In this case, among theplurality of array antennas that compose the reception antenna 3, atleast three of the array antennas are arranged such that their phasecenter points form an isosceles triangle. In addition, a plurality ofsets of the reception antenna 3 may be provided. In addition, the axesof the array antennas 31, 32 and 33 are not necessarily required to eachbe along the vertical direction. The effects of the invention aredemonstrated as long as the array antennas of a plurality of arrayantennas provided by the reception antenna 3 are arranged such that thephase center points of three adjacent array antennas form an isoscelestriangle, regardless of the direction of the isosceles triangle (whetherthe apex or base is on the upper side). In addition, the transmissionantenna 11 and the reception antenna 3 are not required to lie withinthe same plane. Moreover, although the reception antenna 3 and each ofthe reception element antennas 30 were indicated to be lying in the sameplane in the explanation of FIG. 1, a level difference may be providedthere between in consideration of design. In addition, the plurality ofantennas that compose the reception antenna 3 are not limited to arrayantennas, but rather other types of antennas may be used. In addition, arange of possible values of the phase difference in a verticaldirection, which is a phase difference between phases of radio wavesrespectively received by two adjacent antennas among the three antennasmay be determined based on a positional relationship of the threeantennas, and the phase difference between phases of radio wavesrespectively received by two adjacent antennas among the three antennasmay be corrected according to whether or not the phase difference in thevertical direction determined based on phases of the radio wavesrespectively received by the three antennas is a value within the rangeof possible values.

1. A radar device comprising: a reception antenna that receives radio waves, the radar device detecting an arrival direction of the radio waves by a phase monopulse method, wherein: the reception antenna includes at least three antennas, the three antennas are arranged so that phase center points thereof form an isosceles triangle, the radar device further comprising phase correction means for correcting a phase difference between phases of radio waves respectively received by two adjacent antennas among the three antennas based on a relationship of phases of radio waves respectively received by the three antennas, a distance between the phase center point of the center antenna among the three antennas and the phase center point of at least one of the remaining two antennas is larger than one-half the wavelength λ of the radio waves, a distance a between the phase center point of the center antenna among the three antennas and each of the phase center points of the remaining two antennas in a direction along the base of the isosceles triangle is one-half or less the wavelength λ of the radio waves, a distance b between the phase center point of the center antenna and the phase center points of the remaining two antennas in a direction perpendicular to the base of the isosceles triangle is one-fourth or less the wavelength λ of the radio waves, and in the case the absolute value of the sum of a phase difference ψ21 (−π≦ψ21≦π), which is the phase difference between the phase of radio waves received by one of the remaining two antennas among the three antennas and the phase of radio waves received by the center antenna, and a phase difference ψ31 (−π≦ψ31≦π), which is the phase difference between the phase of radio waves received by the other of the remaining two antennas and the phase of the radio waves received by the center antenna, is larger than 2×b ×2π/λ, the phase correction means corrects one of the phase difference ψ21 and the phase difference ψ31, whose absolute value is larger than that of the other of the phase difference ψ21 and the phase difference ψ31.
 2. The radar device according to claim 1, wherein, in the case of correcting one of the phase difference ψ21 and the phase difference ψ31, whose absolute value is larger than that of the other of the phase difference ψ21 and the phase difference ψ31, the phase correction means corrects the phase difference by subtracting 2π from the phase difference if the value of the phase difference to be corrected is positive, and corrects the phase difference by adding 2π to the phase difference if the value of the phase.
 3. The radar device according to claim 2, wherein the direction along the base of the isosceles triangle is substantially a horizontal direction.
 4. The radar device according to claim 1, wherein the direction along the base of the isosceles triangle is substantially a horizontal direction. 